"Alexander Grigoriev" wrote in message
link.net...
So it can tell semiconductor junctions of an MP3 recorder from
semiconductor
junctions of non-recording devices?

Th OP question was detection not *any* semiconductor device, but a MP3
recorder. By the way, the recorder doesn't have to be in that room. Enough
to have a connected cellphone in a pocket.

Yep, and that can be detected.
What you do with the knowledge that the person has an electronic device that
may or may not record is up to you and what you want to do.